In U.S. Pat. No. 4,255,717, Scifres et al. disclose a monolithic laser device in which interconnecting lateral waveguides are provided to deflect and directly couple light propagating in one or more adjacent spatially displaced emitting segments. Waveguiding may be provided for both the emitting segments and the interconnecting lateral waveguides by a refractive index change due to an injected charge distribution determined by current confining channels or contact stripes or due to material thickness or composition change. The interconnecting branching waveguides, termed the "Y-junction region" because of the Y-shaped branching junctions connecting the interconnecting waveguides with the parallel emitting segments, are used to attenuate any out-of-phase components from propagating. The Y-junction region results in boundary conditions on the phase component to be in-phase.
It has been discovered that in order for the Y-junction region to eliminate the out-of-phase modes, the waveguide structure must be strong enough to suppress evanescent coupling in the Y-junction region. The evanescent coupling, or optical wave overlap between adjacent cavaties, tends to result in the propagation of out-of-phase mode. Once the light propagates away from the Y-junction region there are no restrictions on the relative phase of the adjacent emitters except at the emitting facet. Slight variations in the processing or growth of the wafer can therefore result in phase errors of the light as it propagates along the waveguides. The far field pattern as a result is broadened due to the phase randomization away from the Y-junction region.
It is an object of the present invention to provide a method of producing semiconductor lasers with waveguides which eliminate this phase randomization and thereby result in a diffraction limited far field pattern.